Optical sensor for odometry tracking to determine trajectory of a wheel

ABSTRACT

An optical sensor system for determining trajectory of a wheel includes: a wheel mounted in a wheel arch having an outer surface covered with evenly-spaced wheel treads; an optical sensor mounted in the wheel arch but not touching the wheel, for performing a plurality of counts corresponding to respectively capturing a plurality of images of the wheel according to the wheel treads, and comparing the captured images with a reference image to determine a 2D displacement. The optical sensor further performs a calculation to convert the measured 2D displacement of the wheel from its original position into a distance the wheel travels along a path in order to determine the wheel trajectory.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to optical sensors, and more particularly,to an optical sensor which can be used to perform odometry tracking.

2. Description of the Prior Art

Optical sensors, such as those commonly used in a computer mouse, candetect miniscule changes in direction in order to track the motion of anobject over a 2D surface. Optical sensors work by illuminating thesurface on which the object moves to capture an image, and comparing areference image and the captured image in order to determine how farfrom the origin the object has moved. This image comparison generatesaccumulated delta y and delta x values ; computer algorithms can then beused to determine the resultant motion of the object.

The advantage of optical sensors is that only a single sensor is neededto determine angular motion, as the optical sensor can generate bothdelta x and delta y values. Optical sensors are typically used inapplications where only small distances need to be determined, however.If an optical sensor could be implemented in an application which movesvia the use of wheels, the optical sensor could track the motion of thewheels and then convert the detected motion to real-life distance.

It is therefore an objective of the present invention to employ a singleoptical sensor for tracking motion of a wheel in order to performdistance and odometry tracking.

SUMMARY OF THE INVENTION

An optical sensor system for determining trajectory of a wheelcomprises: a wheel mounted in a wheel arch having an outer surfacecovered with evenly-spaced wheel treads; an optical sensor mounted inthe wheel arch but not touching the wheel, for performing a plurality ofcounts corresponding to respectively capturing a plurality of images ofthe wheel according to the wheel treads, and comparing the capturedimages with a reference image to determine a 2D displacement. Theoptical sensor further performs a calculation to convert the measured 2Ddisplacement of the wheel from its original position into a distance thewheel travels along a path in order to determine the wheel trajectory.

A method for determining trajectory of a wheel comprises: mounting awheel in a wheel arch, the wheel having an outer surface covered withevenly-spaced wheel treads; utilizing an optical sensor mounted in thewheel arch but not touching the wheel to perform the following steps:capturing a plurality of images of the wheel according to the wheeltreads to generate a plurality of counts, respectively; comparing thecaptured images with a reference image to determine a 2D displacement ofthe wheel; and performing a calculation to convert the measured 2Ddisplacement of the wheel from its original position into a distance thewheel travels along a path in order to determine the wheel trajectory.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of an optical sensor mounted above a wheel.

FIG. 2A is an illustration of accumulated motion generated by a wheelmoving in a straight direction.

FIG. 2B is an illustration of accumulated motion generated by a wheelmoving in an angular direction.

FIG. 3A is an illustration of generated angular motion of the wheelillustrated in FIG. 2.

FIG. 3B is an illustration of trajectory of the wheel illustrated inFIG. 2 being the front wheel of a car.

DETAILED DESCRIPTION

An exemplary embodiment of the present invention uses an optical sensorpositioned above a wheel, as illustrated in FIG. 1. The odometrytracking system 100 comprises a wheel 150 mounted in a wheel arch 120.Three optical sensors 131, 161 and 191 are illustrated in the diagram,being positioned at the top of the wheel arch, at the right side of thewheel arch and at the left side of the wheel arch, respectively. Eachoptical sensor is protected by a respective casing 133, 163, and 193. Itis noted that an exemplary embodiment of the present invention onlyrequires a single optical sensor of the illustrated optical sensors 131,161, 191 in order to perform odometry tracking. The three sensors areillustrated in order to give examples as to possible placement, but notto limit the invention.

The casings 133, 163, 193 are provided in order to protect therespective optical sensor 131, 161, 191 from damage. These casings canbe clear housings that are flush with the wheel arch 120 or protrude.The aim of the casings 133, 163, 193 is to protect the optical sensor131, 161, 191 from damage. Further, when the optical sensor 131, 161,191 is used to determine motion of a wheel in a car, the casing 133,163, 193 can also protect it from splashes etc.

By using one of the optical sensors 131, 161, 191, an accuratedetermination of how far the wheel 150 has travelled, as well as thetrajectory of the wheel 150, can be estimated. As detailed above, theoptical sensors 131, 161, 191 are mounted on the top of the wheel arch120. The wheel arch 120 could be a wheel arch of a motorized vehiclesuch as a car, or a wheel arch in a treadmill. As the wheel 150 rotates,the optical sensors 131, 161, 191 generate reports based on a number oftreads which are imaged.

Refer to FIG. 2A and FIG. 2B, which illustrate how the reports/counts ofthe optical sensor generate accumulated motion parameters. FIG. 2Aillustrates the generated accumulated motion when the wheel 150 rotateswithout turning. As shown in the diagram, no x values are generated buty values are generated in the opposite direction from the wheel motion.These accumulated values can be termed Dy.

FIG. 2B illustrates the generated accumulated motion when the wheel 150rotates and turns at the same time. As shown in the diagram, both x andy values are generated in the opposite direction from the wheel motion.In order to determine the resultant distance of the wheel 150, thehypotenuse of Dx and Dy must be calculated. This value can then betranslated into a real-world distance.

A calibration step generates a ratio that can be used for conversion.The calibration process is performed to calculate how far the wheelturns for each count of the sensor. As noted above, the countcorresponds to a sensor tread of the sensor. Assuming the wheel rotatesas illustrated in FIG. 2A, then it can be calculated how far in realterms the wheel turns because the circumference, C, of the wheel is aknown value.

The circumference of the wheel can be calculated using the Pythagoreanequation: C=2πr

As the wheel rotates, delta y values are accumulated until Dycorresponds to one rotation of the wheel. The accumulated value Dy has adirect relationship to C. It is determined how many reports/counts thereare in Dy, and this value is used to divide the circumference C in orderto generate a distance per count (dpc). This is illustrated by thefollowing equation:

$\begin{matrix}{{dpc} = {\frac{c}{Dy} = \frac{2\pi \; r}{Dy}}} & (1)\end{matrix}$

A trajectory of the wheel 150 is then determined. If the optical sensor131, 161, 191 only plots a change in the y direction, i.e. only delta yvalues are generated, then the wheel 150 is determined to be rotatingwithout turning and a simple conversion of counts can be used togenerate the distance travelled by the wheel 150. If, however, the wheel150 is both turning and rotating then the angle 0 of the wheel turn canbe calculated using simple trigonometry, as illustrated in FIG. 3A andshown by the following equation:

$\begin{matrix}{{\tan \; \theta} = \frac{Dx}{Dy}} & (2)\end{matrix}$

Once the turning angle of the wheel 150 is determined, a trajectory ofthe wheel 150 can be plotted, as illustrated in FIG. 3B. FIG. 3B is adiagram of the odometry system 100 being a car with four wheels. Thefront right-hand wheel is the wheel 150 illustrated in FIG. 3B. As theturning angle θ of the wheel 150 is known, the internal angle betweenthe wheel and the side of the car can be calculated by using rightangles i.e. 90°-θ.

A perpendicular line to the turned wheel 150 will intersect with anextended line from the rear axles of the car 100 to form a right-angledtriangle having sides L, R and E. L is the length of the car 100 andtherefore is a known value. Using trigonometry, the length of R and Ecan be calculated, as illustrated by the following equations:

$\begin{matrix}{R = \frac{L}{\sin \; \theta}} & (3)\end{matrix}$

$\begin{matrix}{E = \frac{L}{\cos \; \theta}} & (4)\end{matrix}$

As illustrated by the dotted lines, the car 100 will move along a curvehaving a radius R from point O. By using the optical sensor 131, 161,191 to determine a rotated distance of the wheel 150 and converting thatdistance into real-world values, a total distance d moved along thecurve by the car 100 can be calculated.

As detailed above, a distance per count has been calculated in thecalibration stage. This value can be used to calculate a real distancetaken by the vehicle 100. When the vehicle 100 moves in a straightdirection i.e. no change in x, the values can be directly put intoequation (1) by multiplying a number of counts (treads) with thedistance per count. In effect, this converts a distance monitored by theoptical sensor 131, 161, 191 into a real distance. This is shown belowas equation (5) :

Distance=Dy×dpc

If the vehicle 100 is turning, the displacement measured by the opticalsensor 131, 161, 191 is calculated number by using the hypotenuse of Dxand Dy. This value is then converted into counts, and is multiplied withthe value dpc to determine a distance travelled along the curve. This isshown below as equation (6) :

Distance=√{square root over (Dx ² +Dy ²)}×dpc

The final stage in the calculation places this determined distance onthe curve calculated in FIG. 3B. In this way, a trajectory of a wheelcan be calculated to high accuracy.

Although the above is described using a car as an exemplary embodiment,it should be appreciated that the concept can be applied to anyapplication which tracks the motion of a wheel. Further, the opticalsensor can also be calibrated to determine a vertical distance from thewheel, so that if air pressure of the wheel changes or some otherfactors cause the distance between the optical sensor and the wheel tochange (the vehicle moves over rocky terrain, for example) the change indistance can be compensated for.

No matter what implementation the optical sensor is applied to, thewheel radius should be set as a known parameter in an initializationprocedure. In the example provided in FIG. 3B, the length of the car Land the distance between the rear axles W will be known values and canalso be set as the initialization parameters. This is not a limitationof the invention.

To summarize, the present invention provides an optical sensor which canutilize changes in 2D motion of a wheel to determine angular motion ofthe wheel. By plotting a trajectory of the wheel using the determinedchange in motion, a distance the wheel moves along said trajectory canalso be determined.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

What is claimed is:
 1. An optical sensor system for determiningtrajectory of a wheel, the optical sensor system comprising: a wheelmounted in a wheel arch having an outer surface covered withevenly-spaced wheel treads; and an optical sensor mounted in the wheelarch but not touching the wheel, for performing a plurality of countscorresponding to respectively capturing a plurality of images of thewheel according to the wheel treads, and comparing the captured imageswith a reference image to determine a 2D displacement; wherein theoptical sensor further performs a calculation to convert the measured 2Ddisplacement of the wheel from its original position into a distance thewheel travels along a path in order to determine the wheel trajectory.2. The optical sensor system of claim 1, wherein the optical sensorfirst performs a calibration process by measuring a 2D displacement ofthe wheel corresponding to a single rotation of the wheel, determineshow many counts are performed according to this single rotation, thenutilizes a circumference of the wheel to determine a distance per count(dpc) value, wherein the dpc value is used to convert the measured 2Ddisplacement into a distance the wheel travels along a path.
 3. Theoptical sensor system of claim 2, wherein the calibration process isperformed when the wheel rotates but does not turn.
 4. The opticalsensor system of claim 1, wherein the optical sensor determines thewheel trajectory by calculating a turning degree of the wheel accordingto a trigonometric manipulation of the captured 2D displacement, andutilizes the calculated turning degree and at least one other parameterof the optical sensor system to determine a turning curve along whichthe wheel moves.
 5. optical sensor system of claim 4, being a car,wherein the at least one other parameter of the optical sensor system isa distance between a front wheel axle and a rear wheel axle of the car,and the turning curve is centered on a point where an extended line fromthe rear axle crosses a perpendicular line to the front axle.
 6. Theoptical sensor system of claim 1, being a treadmill.
 7. A method fordetermining trajectory of a wheel, the method comprising: mounting awheel in a wheel arch, the wheel having an outer surface covered withevenly-spaced wheel treads; utilizing an optical sensor mounted in thewheel arch but not touching the wheel to perform the following steps:capturing a plurality of images of the wheel according to the wheeltreads to generate a plurality of counts, respectively; comparing thecaptured images with a reference image to determine a 2D displacement ofthe wheel; and performing a calculation to convert the measured 2Ddisplacement of the wheel from its original position into a distance thewheel travels along a path in order to determine the wheel trajectory.8. The method of claim 7, wherein the optical sensor first performs acalibration process comprising the following steps: measuring a 2Ddisplacement of the wheel corresponding to a single rotation of thewheel; determining how many counts are performed according to thissingle rotation; and utilizing a circumference of the wheel to determinea distance per count (dpc) value; wherein the dpc value is used toconvert the measured 2D displacement into a distance the wheel travelsalong a path.
 9. The method of claim 8, wherein the calibration processis performed when the wheel rotates but does not turn.
 10. The method ofclaim 7, wherein the step of determining the wheel trajectory comprises:calculating a turning degree of the wheel according to a trigonometricmanipulation of the captured 2D displacement; and utilizing thecalculated turning degree and at least one other parameter to determinea turning curve along which the wheel moves.
 11. The method of claim 10,being used in a car, wherein the at least one other parameter is adistance between a front wheel axle and a rear wheel axle of the car,and the turning curve is centered on a point where an extended line fromthe rear axle crosses a perpendicular line to the front axle.
 12. Themethod of claim 7, being used in a treadmill.
 13. A car navigationsystem, comprising: a car, comprising: a wheel mounted in a wheel archof the car; and an optical sensor mounted in the wheel arch but nottouching the wheel, for capturing a plurality of images of the wheel,and comparing the captured images to determine a 2D displacement;wherein the optical sensor further performs a calculation to convert themeasured 2D displacement of the wheel from its original position into adistance the wheel travels along a path in order to determine the wheeltrajectory.
 14. The car navigation system of claim 13, wherein the wheelhas an outer surface covered with evenly-spaced wheel treads, and theplurality of images are captured according to the wheel treads.